Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

24. Enolate Chemistry: Reactions at the Alpha-Carbon

Tautomers of Dicarbonyl Compounds

Organic Chemistry

24. Enolate Chemistry: Reactions at the Alpha-Carbon

Tautomers of Dicarbonyl Compounds

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Problem 65

Textbook Question

Pentane-2,4-dione (acetylacetone) exists as a tautomeric mixture of 8% keto and 92% enol forms. Draw the stable enol tautomer, and explain its unusual stability.

Verified step by step guidance

Understand the concept of tautomerism: Tautomerism is a chemical equilibrium between two structural isomers (tautomers) that differ in the position of a proton and the double bond. In this case, pentane-2,4-dione (acetylacetone) exists in keto and enol forms.

Draw the keto form of pentane-2,4-dione: The keto form has two carbonyl groups (C=O) at positions 2 and 4 of the pentane chain. Represent this structure as CH3-CO-CH2-CO-CH3.

Draw the enol form of pentane-2,4-dione: The enol form is created by transferring a proton from the central methylene group (-CH2-) to one of the oxygen atoms of the carbonyl group, forming a hydroxyl group (-OH) and a double bond between the central carbon and the adjacent carbon. Represent this structure as CH3-C(OH)=CH-C(O)-CH3.

Explain the unusual stability of the enol form: The enol form of acetylacetone is stabilized by intramolecular hydrogen bonding. The hydroxyl group (-OH) forms a hydrogen bond with the oxygen atom of the adjacent carbonyl group, creating a six-membered ring structure. This hydrogen bonding significantly lowers the energy of the enol form, making it more stable than expected.

Discuss the dominance of the enol form: Due to the stabilization provided by intramolecular hydrogen bonding, the enol form is the predominant tautomer in the equilibrium mixture, accounting for 92% of the total, compared to only 8% for the keto form.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Tautomerism

Tautomerism is a chemical phenomenon where two or more structural isomers, known as tautomers, exist in dynamic equilibrium. In the case of pentane-2,4-dione, the keto form (with a carbonyl group) and the enol form (with an alcohol group) are tautomers. The equilibrium between these forms can shift based on conditions such as pH and solvent, influencing the stability of each tautomer.

Enol Stability

The enol form of a compound can exhibit unusual stability due to intramolecular hydrogen bonding and resonance stabilization. In pentane-2,4-dione, the enol form benefits from a hydrogen bond between the hydroxyl group and the carbonyl oxygen, which stabilizes the structure. This intramolecular interaction lowers the energy of the enol form, making it more favorable despite being less common than the keto form.

Keto-Enol Equilibrium

Keto-enol equilibrium refers to the balance between the keto and enol forms of a compound. Factors such as solvent effects, temperature, and the presence of substituents can influence the position of this equilibrium. In pentane-2,4-dione, the predominance of the enol form (92%) over the keto form (8%) indicates that the enol is significantly stabilized, highlighting the importance of molecular interactions in determining the stability of tautomers.

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